So far, due to the excellent physical and chemical properties, good processability, and outstanding cost performance, polyethylene is widely applied to various fields as one of the most important materials. It can be processed into articles in various shapes by using various molding methods, such as extrusion molding, blow molding, injection molding, film molding, rotational molding and the like. Typical examples of articles of this kind include tube, drum, bottle, box, film, piece, etc. However, polyethylene materials having different physical properties and processabilities are required for various articles, so as to obtain qualified ones. Some special articles, for example, tube, bottle and film, require the polyethylene to have not only excellent physical property and processability, but also a good balance between the two properties. Thus, stringent requirements on the polyethylene material greatly increase difficulty in microstructure design of the polyethylene material, especially in material composition and molecular structure design.
The molding of polyethylene articles started from the 1950s, which has prominent advantages for molding large and hollow articles, for example, the articles have no wield marks, no residual stresses, and the moulds have a low cost. Usually, the rotational molding method comprises the following steps: (1) feeding: feeding the plastic resin powders into a hollow mould; (2) heating: heating the hollow mould to melt the resin powders therein, while rotating the hollow mould so as to take advantage of the centrifugal force to make the melted resin powders stick closely to the inwall of the rotating hollow mould; accordingly, the molding and self-compaction are achieved; (3) cooling: cooling the hollow mould and the shaped resin article by means of such cooling mediums as air and/or water; (5) demolding: taking out the cooled article. For the details of the abovementioned rotational molding, please refer to “ROTATIONAL MOLDING TECHNOLOGY,” 2001, pp. 2-13, as well as U.S. Pat. No. 4,029,729 and U.S. Pat. No. 4,115,508.
Obviously, in all the polyethylene articles, the molding of a large and hollow article is the most difficult, particularly for the rotational molding, because the rotational molding requires a very good balance between the physical properties and the processing properties of a polyethylene composition. However, a lot of known polyethylene raw materials, in particular conventional non-crosslinked polyethylene compositions, cannot completely meet the abovementioned strict requirement. A rotationally-molded crosslinked resin article has a series of important advantages, such as a relatively high heat distortion temperature (HDT), a relatively high tensile strength, a relatively low thermal expansion coefficient, a relatively strong environmental stress cracking resistance (ESCR), excellent weather resistance and outstanding chemical solvent resistance, etc.
Crosslinking agents commonly applied to rotational molding polyethylene compositions are various peroxides. An ideal crosslinking agent shall enable a polyethylene composition to have a suitable crosslinking retardation time (scorch time) so as to avoid impact on processing and formation by premature crosslinking of the composition, and, meanwhile, it shall broaden the safety processing window of the composition. However, the vast majority of the abovementioned peroxide crosslinking agents do not possess such characteristics, so the effect thereof is unsatisfactory.
To solve that technical problem, one solution is to add an aid to a crosslinked resin composition using a peroxide as crosslinking agent to improve the crosslinking retardation time. DE2553145 and DE2553094 disclose the content of using a mixture of peroxides of different crosslinking retardation times. However, a disadvantage of doing that is the time of curing (maintaining) the crosslinked resin composition becomes longer, which is very disadvantageous for shortening the production time. Furthermore, due to the carcinogenic risk of amine-based aids, they are now not allowed to be used any more.
It is known that quinhydrone and antioxidants can be used as crosslinking retarders for crosslinked resin compositions which employ peroxides as crosslinking agents. However, the addition of quinhydrone and antioxidants leads to a decrease of δ(delta) torque value after crosslinking, and further impacts the final properties of the cured article. As to the technical details hereinabove, reference can be made to U.S. Pat. No. 5,292,791 and U.S. Pat. No. 5,245,084.
Another method to solve the abovementioned technical problem is to use an organic peroxide with a relatively long half-life period as the crosslinking agent. WO2006/066984 discloses that an organic peroxide of trioxepanes has a relatively half-life period, but the crosslinking efficiency is relatively low when said peroxide is used alone, which cannot meet the requirement of production and use. All the abovementioned reference documents are incorporated here in entirety for reference.
Therefore, the present invention aims to find a crosslinked composition system that can significantly improve or raise the safety processing temperature of a crosslinked polyethylene composition, so as to avoid premature crosslinking of the crosslinked polyethylene composition, and, meanwhile, so that the composition possesses a relatively long crosslinking retardation time, a relatively high crosslinking degree and crosslinking efficiency.